Using standard HTML, your choice for border is limited, & can be pretty boring! You can change the color, the thickness, & that is about it.
However, with the use of stylesheets, you have much more of a variety of borders to implement onto your webpage.

There are three css settings you can change on your borders:

border-width
border-color
border-style

Most of the time you will only affect these attributes with 1 simple reference to the border attribute, however it is worth noting you can set these attributes individually.
As well, you will override anything that you have previously defined in a stylesheet since there is no order rule for these attributes.

Here's an example:

<div style="border: 3px coral solid;">text</div>

To affect specific sides, use the following attributes:

border-top
border-left
border-bottom
border-right

Using these you can force individual borders to display differently, or not at all.